Pancreatic α-amylase is an enzyme in the digestive system, catalyzing the initial step in the hydrolysis of starch, a principal source of glucose in the diet. It has been demonstrated that the activity of human pancreatic α-amylase (HPA) in the small intestine correlates to post-prandial glucose levels, the control of which is an important factor in diabetes and obesity. Salivary α-amylase is also involved in starch digestion and in the maintenance of the bacteria involved in oral plaque formation. Thus, modulation of α-amylase activity through the therapeutic use of inhibitors is of considerable medical relevance. Although two α-glucosidase inhibitors, acarbose (Precose™) and miglitol (Glyset™) have been used medically, their effectiveness may be limited by undesired side effects which may be due to non-specific inhibition of other α-glycosidases. These side effects may also be compounded by systemic absorption of these drugs and hence their distribution throughout the body. Unusually for an oral drug, poor absorption is a desirable quality for a pancreatic α-amylase inhibitor since the effect is only required locally (e.g. in the gut or oral cavity) and low systemic availability would reduce unwanted side effects.
Subsequent to filing of the related patent application noted above, it was reported that various flavonoids including myricetin inhibit human salivary α-amylase with IC50 values in excess of about 9 or 10 μM (Lo Piparo, E. et al. “Flavonoids for Controlling Starch Digestion Structural Requirements for Inhibiting Human α-Amylase”; J. Med. Chem.; published on web May 29, 2008). As reported in the latter document, acarbose inhibited human salivary α-amylase with an IC50 of approximately 1 μM. Previously, myricetin was reported as inhibiting porcine pancreatic α-amylase at an IC50 value of 0.38 mM (Taderal, K. et al. (2006) J. Nutr. Sc., Vitaminol 52:149-153).
Asada, Y. et al. (1988) Phytochemistry 27, 1497-1501 described naturally occurring glycosylated compounds containing a myricetin moiety having the following structures.

Montbretin A and B were isolated from a common garden plant known as “Montbretia” which has been used as an anti-tumor remedy in Japanese folk medicine. However, no biological activity for montbretin A or B was reported.